His win in the Illinois primary does not cement his status as eventual GOP nominee, but it certainly firms it up. The next primary, Saturday in Louisiana, does not favor Mitt Romney, but many of the April contests do.

Washington — Mitt Romney won the Illinois primary handily – both in the popular vote and in the delegate count – solidifying his status as the prohibitive favorite for the Republican presidential nomination.

Mr. Romney’s superior organization and financial firepower, and a growing reputation as the most electable Republican in the field, sealed the deal in Illinois, a state whose GOP voter profile played to Romney’s strengths. Sixty percent of Tuesday’s voters said Romney was the most likely candidate to beat President Obama in November, higher than the average of exit polls in earlier contests.

“An improved sense that he understands voters’ problems boosted Mitt Romney to victory in the Illinois primary, as did a less religiously focused, less strongly conservative electorate than he’s faced in other contests, especially to the south,” writes Gary Langer, pollster for ABC News, in an analysis of the Illinois exit poll. “But a shortfall among less well-heeled Republicans marks his continued challenges.”

With 99 percent of the vote counted early Wednesday, Mr. Romney took 46.7 percent, Rick Santorum had 35 percent, Ron Paul had 9.3 percent, and Newt Gingrich had 8 percent. Nobody dropped out of the race, but Romney supporters hoped his double-digit victory in a big state would quiet talk of a contested GOP convention in August.

Romney won at least 43 of the 54 Illinois delegates up for grabs Tuesday, and Mr. Santorum won at least 10, according to the Associated Press. In the overall AP delegate count, Romney leads with 563 out of the 1,144 required to secure the nomination. Santorum has 263, Mr. Gingrich has 135, and Congressman Paul has 50.

The next contest, Saturday’s primary in Louisiana, plays more to Santorum’s advantage. The Real Clear Politics average of recent polls there shows Santorum leading by 8.5 percentage points. But in April, the playing field tilts again toward Romney, the former governor of Massachusetts, with seven contests in Northern states. Two big tests loom for Santorum: the April 3 primary in Wisconsin, where the two recent polls show Santorum strong, and the April 24 primary in Pennsylvania, which Santorum represented first in the House and then the Senate for total of 16 years, until 2007.

If Santorum fails to do well in Wisconsin – either winning outright or coming close, as he did in Michigan and Ohio – and then fails to win Pennsylvania, calls for him to exit the race are likely to escalate. Establishment Republicans, who tend to favor Romney, have been treading gingerly on the Santorum question, as they do not want to alienate conservative and evangelical Republicans. Turnout among that key demographic in November will be crucial for the party’s nominee.

In his victory speech Tuesday night, Romney spoke as the likely nominee, training his sights on Mr. Obama.

“It’s time to say this word: enough. We’ve had enough,” Romney said, speaking of Obama’s stewardship of the economy.

But his message could just as easily have applied to his Republican primary competitors – both Santorum and Gingrich, the former House speaker – who are increasingly seen as spoilers rather than as serious competitors for the nomination. Given the delegate math, their only scenario for victory involves depriving Romney of a majority of delegates before the convention in Tampa, Fla., forcing a contest there.

The history of modern contested conventions has not been kind to either party. Romney himself has said that if the nomination race goes all the way to Tampa, the effort to replace Obama will be doomed. Obama faces no challenge for the Democratic nomination, allowing him and his campaign to raise money and focus solely on the GOP while the Republicans continue to attack one another. Gingrich’s only statement to voters Tuesday night was a communiqué that did not congratulate Romney, instead going negative on his money advantage in the primaries.

“To defeat Barack Obama, Republicans can’t nominate a candidate who relies on outspending his opponents 7-1,” Gingrich said. “Instead, we need a nominee who offers powerful solutions that hold the president accountable for his failures.”

If Gingrich fails to do well in Louisiana, any path forward for his candidacy will have vanished, if it hasn’t already. His only two primary victories have been in South Carolina and Georgia.

Santorum signaled his commitment to Pennsylvania by delivering his remarks Tuesday night in Gettysburg – the setting of Illinois native son President Lincoln’s most famous address. In his remarks, Santorum emphasized his family’s immigrant, working-class roots in Pennsylvania.